There is known a hybrid work machine including a generator motor (motor generator) that is mechanically coupled to both an engine and a hydraulic pump, and an electrical storage device (electrical storage means) such as a lithium ion battery or a capacitor (see, e.g., Patent Literature (PTL) 1). In the work machine disclosed in PTL 1, the hydraulic pump is driven by the engine and the generator motor. When a hydraulic oil delivered from the hydraulic pump is supplied to actuators such as hydraulic cylinders and hydraulic motors, a boom, an arm, a bucket, an upperstructure, etc. are driven.
PTL 1 discloses a technique of limiting a powering torque value depending on a residual charge amount (charge rate), and controlling the generator motor to output the limited powering torque value, when the residual charge amount is not more than a predetermined value set in advance. PTL 1 further discloses that, when the residual charge amount is reduced, a maximum value of input torque to the hydraulic pump is controlled such that output power of the hydraulic pump is held constant.